Bistable deformable mirror devices (DMDs) are the subject of the above-listed copending U.S. patent application entitled "Spatial Light Modulator" Ser. No. 07/408355. There are many uses for such devices, such as, for example, as a projection light valve for high-definition television (HDTV). For such a system, DMD pixel arrays as large as 1.8 megapixels are required, addressed by an underlying CMOS address circuit.
In order to minimize development costs and maximize chip yields, it is important to choose a well established, production tested CMOS technology. High volume CMOS wafer fabricators are reluctant to customize their CMOS process for a particular user's application. For this reason, it is necessary that the DMD superstructure place no special requirements on the CMOS process.
Unfortunately, there are several process artifacts in the CMOS multi-level metalization process that can cause yield losses in the DMD superstructure. These artifacts include aluminum hillocks, pinholes in the protective oxide, a nonplanar chip surface and protective oxide contacts with a steep sidewall angle.
An example of a DMD device is shown in above-listed patent application, Ser. No. 07/355049 entitled "Spatial Light Modulator" filed May 15, 1989. As shown in the aforementioned application, the DMD device has a beam reflective surface which is deformed, or pivoted, into contact with a landing pad or electrode under control of voltages selectively applied either by the beam or to a control electrode or to both.
Aluminum hillocks can cause weak spots in the overlying protective oxide, leading to dielectric breakdown when high voltage reset pulses, which are necessary with DMD system arrays, are applied to the overlying landing electrodes.
An additional problem is that the hillocks are replicated by the electrode metal and when hillocks are under the DMD address control electrodes, the DMD torsion beam can land on these hillocks, causing the address electrodes to be shorted to the beam. When hillocks are under the landing electrode, the beam is prevented from rotating to the full deflection angle, thereby reducing the light-to-dark effect of the deforming beam.
In addition, pinholes in the protective oxide cause shorts between the electrode metal and the final CMOS metalization layer, thereby further reducing the yield of good devices and also reducing their life expectancy.
The final CMOS metalization layer containing the electrical circuits is not planarized and it produces steps in the protective oxide which can lead to residual filaments when the electrode metal is etched with an anisotropic plasma etch. Thus, filaments cause shorts between neighboring electrodes.
Finally, the protective oxide contacts in a standard CMOS process are not etched with sloped sidewalls, but rather tend to be nearly vertical. This can lead to poor step coverage of the electrode metal down into the contacts.
A need therefore exists in the art for a DMD device which is constructed integral with a CMOS substrate without being effected by the typical CMOS artifacts.